1. Field of the Invention
Embodiments of the inventive concept relate to an analog-to-digital converter, an image sensor including the same, and an apparatus including the image sensor.
2. Description of the Related Art
In general, an electronic system such as an image sensor system or biosensor system includes an analog-to-digital converter (hereinafter, referred to as ‘ADC’) installed in every column in order to read a large amount of data at high speed.
Recently, demand for an ADC requiring high speed operations, high resolution, and low power consumption has increased due to the development of an image sensor such as a complementary metal oxide semiconductor image sensor (CIS), a charge coupled device (CCD), or the like, as well as the development of various communications technologies.
As generally used ADCs, there are various types of ADCs including but not limited to a sigma-delta ADC, a cyclic ADC, and the like.
Since a sigma-delta ADC has noise-shaping characteristics, it can be used as an ADC having adequate high-resolution. However, since the sigma-delta ADC requires a large number of clocks for the operation thereof, there is a need to operate an operational amplifier having an integrator configured within the sigma-delta ADC at high speeds in order to operate the sigma-delta ADC at a high speed. As a result, in the case in which the sigma-delta ADC is used for operating at a high speed, power consumption thereof may be increased. In addition, due to the number of clocks in the sigma-delta ADC being increased, internal circuits therein may occupy a relatively large area.
In the case of a cyclic ADC, as compared to that of the sigma-delta ADC, the number of clocks necessary for obtaining the same level of resolution is relatively small. Thus, it is easy to implement an ADC able to operate at high speed. However, in the cyclic ADC, in order to reduce an effect of thermal noise generated at the time of sampling, a large capacity sampling capacitor is needed. Therefore, a cyclic ADC requires a relatively large area, and power consumption may also be increased. In order to implement an ADC having a relatively high resolution, the cyclic ADC needs to include an operational amplifier having a large gain.